I'd think it's a deliberate design choice having it this way. If you think about it, paying only for actually used space would mean a lot smaller income for the company.

Maybe it would also erode the magic in it - people would start thinking if they should have this and that files on Dropbox or not. Or accidentally piling up huge bills doing some mistake managing their files...

I don't know about that. Sure, there are some people using 5-10GB of a 50GB account, but I'm sure there are lots of people that are sticking with the free account when they'd gladly pay a little bit $1-$2 for slightly more storage, and people at the other end that are either frustrated by the 100GB limit or haven't even bothered using the service because they know they need to store more than 100GB.

I trust Dropbox has done extensive market research and concluded that having two price tiers, plus the other benefits that come with such simplified pricing, outweigh the flexibility of usage-based pricing.